A new study suggests that taking aspirin may protect against the most prevalent types of breast cancer. Women who took aspirin on a regular basis had a lower risk of breast cancer compared to women who did not take the drug, but it was only effective in preventing tumors that depend on estrogen.

It’s too early to recommend regular aspirin use to prevent breast cancer, according to an editorial that accompanies the report in the Journal of the American Medical Association. But women who take daily aspirin to protect against heart disease may gain the added benefit of preventing certain types of breast cancer, writes Raymond N. Dubois of Vanderbilt University in Nashville, Tennessee.

Mary Beth Terry of Columbia University in New York City, and her colleagues questioned breast cancer patients and women without a history of breast cancer about their use of aspirin, ibuprofen, or acetaminophen. Women who took aspirin occasionally had a 20 percent lower risk of developing the disease, but women who took aspirin at least seven times per week for more than six months had a 28 percent lower risk of breast cancer.

Ibuprofen, which is sold as Advil or Motrin, also prevented breast cancer, but it did so to a lesser extent than aspirin. Acetaminophen, or Tylenol, offered no protection.

The researchers also found that aspirin prevented one class of breast cancer but not another. Estrogen promotes the growth of approximately 70 percent of breast tumors. Aspirin protects against these estrogen-sensitive tumors, but not against tumors that do not depend on estrogen, which are often more aggressive.

Previous studies in mice by Andrew Dannenberg of Cornell University in New York City had shown that aspirin blocks an enzyme called cyclooxygenase, or COX, that the body needs to produce estrogen.

“Our studies in mice predicted that aspirin and similar drugs would only affect tumors that depend on estrogen,” says Dannenberg, a co-author on the study. “But many studies in mice have no relevance in humans. Here is a clear case in which animal studies predict a response in humans.”

More than a dozen studies over the past decade have linked aspirin use to protection against breast cancer, but this is the first study in humans to link its protective effect to estrogen.

“A twenty percent reduction in risk is still a fairly modest effect,” says Terry. “It could well be that aspirin users are different in some other way and that could have explained our results. However, the fact that we only saw the effect among women with hormone-sensitive tumors suggested that the effect is real and aspirin works by reducing estrogen levels.”

Studies that show an association between a cause and an effect are sometimes misinterpreted.

“This could have been just another study showing an association between aspirin use and breast cancer,” says Michael Thun, chief epidemiologist with the American Cancer Society in Atlanta, Georgia, who first discovered a link between aspirin and colon cancer.

“The novel contribution here is that it is a testable mechanism for how aspirin and similar drugs may prevent breast cancer,” Thun says.

Knowing the mechanism by which aspirin and similar drugs prevent breast cancer will help researcher figure out the best way to prevent or treat cancer. Currently several clinical trials are underway to test the effects of more selective anti-inflammatory drugs, such as Vioxx and Celebrex, in treating and preventing breast cancer.

Here’s what needs to be done, before we can recommend these drugs to treat cancer,” says Thun. “We need to define which drugs are truly effective. We need to define the optimal drug dose, the frequency, and duration. And we need to identify the patients in whom the net benefits will outweigh the risk.”

Terry, M.B. et al. Association of frequency and duration of aspirin use and hormone receptor status with breast cancer risk. Journal of the American Medical Association291, 2433-2440 (May 26, 2004).